patch.js

// Simple Script for Patching a JavaScript Function while Runtime
// Copyright 2019 Andreas Fendt. All rights reserved.
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

before = function (i, j) {
    var a = i,
        b = j;
    // this is a function with a comment
    var c = a + b;
    var d = "Hello";
    var e = 'World';
    console.log(d + ' ' + e);
    console.log(c + 3);
}

function patchFunction(func, patch) {
    // helper functions
    function isFunction(functionToCheck) {
        return functionToCheck && {}.toString.call(functionToCheck) === "[object Function]";
    }

    function slashEscape(contents) {
        return contents
            .replace(/\\/g, '\\\\')
            .replace(/"/g, '\\"')
            .replace(/'/g, '\\\'')
            .replace(/\n/g, '\\n')
            .replace(/\r/g, '\\r');
    }

    // check if handlers are functions
    if (!isFunction(func) || !isFunction(patch)) {
        throw "no function";
    }

    var s = func.toString();

    // get offsets of the code
    var offsetArgsStart = s.indexOf("("),
        offsetArgsEnd = s.indexOf(")", offsetArgsStart),
        offsetCodeStart = s.indexOf("{"),
        offsetCodeEnd = s.lastIndexOf("}");

    // get source and arguments of the function
    var source = s.substring(offsetCodeStart + 1, offsetCodeEnd),
        args = s.substring(offsetArgsStart + 1, offsetArgsEnd);

    patchedFunction = patch({source: source, args: args});

    // create helper function which is spawning a new patched function
    var f = "return Function('" +
        patchedFunction.args.replace(" ", "").split(",").join("','") + "','" +
        slashEscape(patchedFunction.source) +
        "')";
    var helper = Function(f)();

    return helper;
}

function patch(func) {
    func.source = func.source.replace("3", "6").replace("a + b", "a - b").replace("Hello", "Welcome");
    return func;
}

after = patchFunction(before, patch);

after(2, 3);